1. Field of the Invention
The present invention relates to an alkaline storage battery using a nickel oxide as a positive electrode, and is suitably applicable to Ni/MH storage batteries using a hydrogen-absorbing alloy capable of electrochemically absorbing and releasing hydrogen as a negative electrode.
2. Descriptiom of Related Art
Recently, with the spread of portable equipment, demand for small secondary batteries has increased. Among them, nickel-cadmium storage batteries using a nickel oxide as a positive electrode and an aqueous alkaline solution as an electrolyte and alkaline storage batteries such as nickel-metal hydride storage batteries have been in great demand because of their merits in cost, energy density and durability.
Of these batteries, the alkaline storage batteries have been able to be further increased in capacity than nickel-cadmium storage batteries by using hydrogen-absorbing alloys capable of electrochemically absorbing and releasing hydrogen as negative electrodes. Positive electrodes of alkaline storage batteries industrialized at present use active materials mainly composed of a nickel oxide. The present invention relates to this nickel positive electrode, and the following explanation is based on nickel-metal hydride storage batteries as a specific example.
One of the problems which must be solved for improving performances of nickel-metal hydride storage batteries is reduction of charging efficiency at high temperatures caused. by generation of heat at the time of charging. That is, in nickel-metal hydride storage batteries, sealing of batteries is attempted by using a reaction which returns to water the oxygen gas generated by a decomposition reaction of water taking place competitively with oxidation (charging) of nickel hydroxide at the time of charging by reducing the oxygen gas with hydrogen contained in the hydrogen-absorbing alloy of negative electrode. For this reason, because of the rising temperature in the battery caused by the heat generated by the reaction, oxygen overvoltage of the nickel positive electrode decreases and energy used for oxidation reaction (charging reaction) of nickel hydroxide decreases to cause sharp reduction of charging efficiency. This phenomenon occurs similarly at the time of charging at high temperatures and also causes deterioration of charging efficiency, resulting in decrease of battery capacity.
Furthermore, the large-sized alkaline storage batteries for electric vehicles which have been developed and partially put to practical use from the viewpoint of environmental protection cannot ensure a battery surface area (radiating surface) which is proportioned to the capacity, and due to the inferior radiating performance, the rise of temperature in the batteries as mentioned above is conspicuous as compared with small-sized batteries. Thus, the above problem becomes more serious.
In order to solve the problems in charging at high temperatures, many proposals to improve charging efficiency at high temperatures have been made.
For example, one of the proposals is to add at least one of the materials having the effect of increasing oxygen overvoltage, such as yttrium, indium, antimony, barium, calcium and beryllium to the surface of active material powders or between the powders (JP-A-5-28992) and another is to similarly add powders of compound of at least one element selected from the group consisting of Ca, Sr, Ba, Cu, Ag and Y which have an average particle size of less than xc2xd times the average particle size of nickel oxide which is an active material (JP-A-8-329937).
Moreover, it has been proposed to add elements having the effect of increasing oxygen overvoltage at the time of charging, such as Ca and Cr in the state of solid solution, to nickel hydroxide which is a positive electrode active material.
However, according to the methods disclosed in the above JP-A-5-28992,JP-A-8-329937 and others, the additives having the effect of increasing the oxygen overvoltage are apt to agglomerate in the positive electrode and the effect of addition is not necessarily sufficient for the amount of the additives. Furthermore, when the elements having the effect of increasing the oxygen overvoltage at charging are incorporated in the active material as a solid solution, the effective capacity of the positive electrode active material decreases according to the amount of the elements incorporated as a solid solution.
The present invention solves the above problems, and the object of the present invention is to provide an alkaline storage battery of high capacity by allowing the components having the effect of increasing oxygen overvoltage at charging to efficiently function to improve not only the charging efficiency at high temperatures, but also the charging efficiency at room temperature, thereby further enhancing discharge capacity of the battery.
For attaining the above object, according to the present invention, in an alkaline storage battery comprising a positive electrode mainly composed of a nickel oxide, a negative electrode, an alkaline electrolyte and a separator, a continuous electrically conductive network is formed of metallic Co and/or a Co oxide and a material having the effect of incearsing oxygen overvoltage on the surface of the nickel oxide of the positive electrode. Thus, dispersibility of the additive is improved to prevent the agglomeration seen in the conventional method, and the oxygen overvoltage is efficiently increased. Furthermore, there is provided an alkaline storage battery of high capacity which is improved in charging efficiency at high temperatures and increased in discharge capacity.